Coated screens

ABSTRACT

Hard-surfacing a screen with a detonation-deposited coating of bonded refractory particles adhered to the surface of the screen and to one another, the coating overlying substantially only the imperforate area of the screen surface, and having a Vickers Hardness of at least 1,000 VPN (with a 300 gm. load) and a thickness at least on the order of 0.003 inch.

United States Patent [151 3,640,760

Nichols, Jr. 1 Feb. 8, 1972 [54] COATED SCREENS [56] References Cited[72] Inventor: George E. Nichols, Jr., Norfolk, Mass. UNITED STATESPATENTS [73] Assignee: Bird Machine Company, South Walpole, 2 1 2 031939 Gallup 1 9 X 3 3,425,900 2/1969 Purdy ..117/99 x 22 F1 d: Dec. 181970 l 1 1 e Primary Examiner-Ralph s. Kendall [21] Appl. No.: 99,535Assistant Examiner-Edward G. Whitby Attorney-Edgar l-l. Kent Related US.Application Data [63] Continuation-impart of Ser. No. 873,334, Nov. 3,1571 ABSTRACT 1969 abandoned" Hard-surfacing a screen with adetonation-deposited coating of bonded refractory particles adhered tothe surface of the [52] US. Cl. ..ll7/99, 1 17/130 R, 210/499, Screenand to one another the coating overlying Substantial), 139/425 245/ l 1only the imperforate area of the screen surface, and having a [51] Int.Cl. ..B44d 5/08, 844d 5/00 vickers Hardness of at least 000 vp (with a300 gm load) [58] Field of Search ..210/499; 139/425 A; 245/ 1; and athickness at least on the order 0003 inch l62/DIG. l; 117/99, 98;209/395 4 Claims, 4 Drawing Figures COATED SCREENS This application is acontinuation-in-part of my pending US. application Ser. No. 873,334filed Nov. 3, 1969, now abandoned.

This invention relates to heavy duty screens employed in high-capacityindustrial separators, centrifuges, refiners, and the like.

Industrial screens, such as those used in the screen bowl centrifuges"described in US. Pat. No. 3,348,767, in the oscillating centrifugesdescribed in US. Pat. No. 3,133,879, and in screening apparatus such asdescribed in U.S'. Pat. No. 3,363,759, have limited useful lives becauseof continual exposure at high rotational or oscillating speeds torapidly flowing particulate matter, which is often also abrasive andcorrosive..Coating such screens, to prolong their useful lives and alsoavoid costly equipment shutdowns, by conventional spraying (up to 600f.p.s.) of refractory materials of requisite hardness and corrosionresistance has proved unsatisfactory because of the inability to achievean operatively thick coating of an economical protective materialwithout plugging or bridging the perforations. Moreover, such coatingsas have been achieved have often been brittle and poorly adherent underthe operational flow conditions.

It is an object of this invention to provide a durable, corrosionresistant, adherent coating for a surface of a screen which issubstantially flat between perforations, the coating overlyingsubstantially all and only the areas between perforations of the screen.

The invention features a screen having a perforate main body portionand, on at least one surface thereof which is sub stantially flatbetween the perforations, a long-wearing detonation-deposited coating ofbonded refractory'particles adhered to this surface and to one another,the coating overlying substantially all and only the imperforate areasof the body portion, and having a Vickers Hardness of at least 1,000 VPNand a thickness at least on the order of 0.003 in.

A preferred process for applying this coating utilizes a method andapparatus such as described, e.g.,'in US. Pat. Nos. 2,714,563 and2,964,420. This process comprises providing a detonatable mixture of afuel gas and an oxidizing gas, and refractory particles unconsumable bythe detonation, igniting the detonatable gaseous mixture to produce adetonation, and transmitting at least some of the detonation energy tothe powdered refractory material to direct that material, at the highvelocity induced by detonation, at the screen surface in a directionsubstantially perpendicular thereto.

Preferably, detonations are produced in an elongated barrel, having anopen end and its axis substantially perpendicular to the screen surfaceto be coated, by repeatedly igniting a charge of detonatable mixtureintroduced into the barrel. Powdered refractory material is repeatedlyintroduced to the barrel downstream of the ignition, thereby to beejected repeatedly from the barrel by the repeated detonations onto thescreen surface until the required coating depth is achieved. Acetyleneand oxygen, in proper proportions, may be used to provide a detonatablemixture, and a preferred refractory material is a tungsten carbidecomposition containing about 9 percent cobalt binder. Utilizing thispreferred material screens have been hard-surfaced to an extent such asto increase their abrasion-wear lifetimes, greatly over chrome-platedscreens, which, in turn, are three to four times as wear resistant asuncoated screens, and yet are with no bridging of openings as fine as0.0l2-inch width slots.

Other objects, features, and advantages will appear from the followingdescription of a preferred embodiment of the present invention, takentogether with the attached drawings thereof, in which:

FIG. 1 is a schematic view of apparatus useful in carrying out theprocess of the present invention; Q

FIG. 2 is a sectional view of FIG. 1, along line 2-2 thereof, showing apreferred orientation of a screen for coating in accordance with thepresent invention.

FIGS. 3 and 4 are sectional and elevational views, respectively, ofother representative screens coated in accordance with the presentinvention.

FIGS. 1 and 2 show detonation-coating apparatus 10 having an elongatedbarrel 12, arranged perpendicular to the perforate surface 14 of ascreen 15, having a plurality of elongated narrow (e.g., 0.0l2-inchslots 16. Barrel [2 has an open end 17. Oxidizing gas is supplied frominlet 18 to chamber 20 and, in bursts controlled by poppet valve 22, tomixing cavity 24, whereas fuel gas is supplied from inlet 26 to chamber28 and, in bursts controlled by poppet valve 30, to mixing cavity 24,valves 22, 30 being operated in unison by cam 32. Purging gas issupplied from inlet 36 to chamber 38, and, in bursts controlled bypoppet valve 40 and cam 42, to mixing cavity 24. A spark plug 46,communicating with ignition chamber 47 of barrel 12, is fired by sparkcoil 48, battery 49, and switch 51, operated by cam 52. A single motor56 may be utilized to drive all three cams 32, 42, 52. Refractory powderis supplied from reservoir 58 at a rate controlled by an appropriatevalve 60, and adheres to imperforate portions only of the perforatedsurface 14 as a hard coating 62.

In operation, cam 32 rotates to open poppet valves 22, 30 to admitcharges of oxidizing gas and fuel gas (e.g., 1 part oxygen to 1-2 partsacetylene by volume) to chamber 24, then further rotates to close thesepoppet valves. lrnmediately after valves 22 and 30 close, cam 42 opensvalve 40 to admit inert purging gas (e.g., nitrogen) to chamber 24,which dilutes the gases remaining around valves 22, 30 to preventflashback into these valves upon detonation. As soonas valve 40 isopened, cam 52 closes switch 51, thereby firing spark plug 15 to ignitethe detonatable gaseous mixture in ignition chamber 47, producing adetonation. Valve 40 remains open to permit purging gas to follow thedetonation combustion products out of the barrel, and then closes topermit the cycle to begin again. Powder may be introduced continuouslyin a slow stream or in bursts timed to coincide with detonation, suchthat powder particles are projected from the open end of the barrel,under acceleration of the detonation, at the perforate surface, to formthereon a detonation-deposited coating 62. A preferred practice of theprocess, for hard-coating, for example, a screen having 0.012 inch wideslots, utilizes finely powdered (10-40 microns, for the most part)particles of a cast composition including tungsten carbide and 9 percentcobalt binder, produced by Union Carbide Corp. under the designationLW-I (AMS 2435)"; a total flow of a detonatable lzl oxygen-acetylenemixture of 360 cu.ft./hr. ignited four times per second in a barrelabout 5 ft. long and 1 inch in diameter; and a purging nitrogen gas flowof I80 cu.ft./hr. The screen surface is maintained about 3 inch from theopen end of the barrel substantially perpendicular to the direction offlow until the desired depth of coating (e.g., 0.005 inch) is produced,having a cross-sectional Vickers Hardness of the order of 1,300 VPN(with a 300 g. load). The screen surface is, as shown, substantiallyflat between the slots (i.e., is planar or curved on a large radius) sothat the perpendicular relationship to the direction of flow may beprovided.

A preferred detonatable mixture utilizes oxygen as oxidizing gas andacetylene as fuel gas, the acetylene being 3.5 percent to 93 percent ofthe mixture by volume. Acetylene-air, hydrogen-air, propane-oxygen, andhydrogen-oxygen also can be combined into detonatable mixtures. Thefrequency of detonation depends on the diameter and length of thebarrel, as well as the cooling system (usually air) utilized. Apreferred purging gas is nitrogen.

A preferred powdery refractory material is a tungsten-carbidecomposition containing, in addition to tungsten carbide, about 9 percentcobalt binder. Somewhat less wear resistant is a tungstencarbide-chromium carbide-nickel composition containing about 5 percentnickel, 25 percent tungsten carbide, and the remainder mixedtungsten-chromium carbides,

LW-S." The resultant coating should be at least in the order of0.003-inch thick, and should have a cross-sectional Vickers tures 66 ofincreasing width between screening surface 67 and opposite surface 73,the apertures being between elongated wires 68 held together by spacedsupports 69, has an overlying detonation-deposited coating 70, havingapertures 71 substantially of width equal to the width of screenapertures 66 at flat screening surface 67. In FIG. 4, a portion of adetonationdeposited coating 80 on the screening surface 82 of aperforated screen 83 is broken away to reveal the substantially equalwidths of theapertures 84 through screen 83 and the apertures 85 throughcoating 80.

Other embodiments will occur to those skilled in them and are within thefollowing claims.

What is claimed is:

1. A screen having a perforate main body portion and, on at least onesurface thereof which is substantially flat between one another, saidcoating overlying substantially all and only the areas betweenperforations of said body portion, and having a Vickers Hardness of atleast 1,000 VPN (with a 300 g. load) and a thickness at least on theorder of 0.003 inch.

2. The screen of claim 1 wherein said refractory particles are of a castcomposition comprising tungsten carbide and a cobalt binder.

of bonded refractory particles adhered to said surface and to 3. Thescreen of claim 1 wherein said main body portion has a plurality ofequally spaced parallel elongated apertures.

4. The screen of claim 1 wherein said body portion has a screeningsurface on one side having smaller apertures than those in the oppositesurface, said apertures increasing in size between said screen surfaceand said opposite surface, and said detonation-deposited coating isadhered to said screening surface only.

, UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.356%,760 Dated February 8, 197 2 Inventor(s) George E. Nichols, Jr.

It is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

Col. 2, line l, after "inch", insert Col. 2, line 51, add -m-- after the"g".

C01,. 2,, line 72, add --m-- after the "g".

Col. l, line 3, add --m-- after the "g Signed and sealed this 20th dayof June 1972.

(SEAL) At'test:

EDWARD M.FLETCHER, JR. ROBERT GOTTSCHALK v Attesting OfficerConmissioner of Patents FORM PO-1QSO (10-69) USCOMM-Dc 60816.:0

i ".5. GOVE'NMINT PRINHNG OFFICE I'II 0-3533

2. The screen of claim 1 wherein said refractory particles are of a castcomposition comprising tungsten carbide and a cobalt binder.
 3. Thescreen of claim 1 wherein said main body portion has a plurality ofequally spaced parallel elongated apertures.
 4. The screen of claim 1wherein said body portion has a screening surface on one side havingsmaller apertures than those in the opposite surface, said aperturesincreasing in size between said screen surface and said oppositesurface, and said detonation-deposited coating is adhered to saidscreening surface only.